Creepy, no? NPR's Robert Krulwich explains that this is science, not the supernatural:
Because this squid was just killed, its muscle cells were still intact and operational. A live squid moves it tentacles by sending an electrical command from its brain to its muscles. The commands say "contract" or "relax." But since this animal lost its head, its brain can't send signals. Salt [from the soy sauce] acts as a substitute.Extra sodium (the salt) sends ions to the cell that trigger the cell to open up, creating a cascade of chemical activities that causes the cell to fire, so the muscle twitches.